Vibrational dephasing mechanisms in liquids and glasses: Vibrational echo experiments

Abstract

Picosecond vibrational echo studies of the asymmetric stretching mode (2010 cm−1) of (acetylacetonato)dicarbonylrhodium(I) [Rh(CO)2acac] in liquid and glassy dibutyl phthalate (DBP) (3.5 K to 250 K) are reported and compared to previous measurements of a similar mode of tungsten hexacarbonyl [W(CO)6]. The Rh(CO)2acac pure dephasing shows a T1 dependence on temperature at very low temperature with a change to an exponentially activated process (ΔE≅400 cm−1) above ∼20 K. There is no change in the functional form of the temperature dependence in passing from the glass to the liquid. It is proposed that the T1 dependence arises from coupling of the vibration to the glass’s tunneling two level systems. The activated process arises from coupling of the high-frequency CO stretch to the 405 cm−1 Rh–C stretch. Excitation of the Rh–C stretch produces changes in the back donation of electron density from the rhodium dπ orbital to the CO π* antibonding orbital, shifting the CO stretching transition frequency and causing dephasing. In contrast, W(CO)6 displays a T2 dependence below Tg in DBP and two other solvents. Above Tg, there is a distinct change in the functional form of the temperature dependence. In 2-methylpentane, a Vogel–Tammann–Fulcher-type temperature dependence is observed above Tg. It is proposed that the triple degeneracy of the T1u mode of W(CO)6 is broken in the glassy and liquid solvents. The closely spaced levels that result give rise to unique dephasing mechanisms not available in the nondegenerate Rh(CO)2acac system.